Carbon dioxide is a well-known gas, which is present in the atmosphere. It is released to the atmosphere in large amounts by fermentation processes, limestone calcinations, and all forms of combustion processes of carbon and carbon compounds. In the recent decades, the attention in respect of said emission has been rising, because of the environmental problem due to future climate change via Greenhouse effect. Consequently, extensive work has been performed over the years in order to develop processes for the removal of carbon dioxide from combustion gases. If possible, a subsequent recovery of carbon dioxide may make those processes economical feasible.
One type of conventional methods for the recovery of carbon dioxide from a gaseous source is the absorption method, in which carbon dioxide is absorbed in an absorbing agent. If other gases, such as oxygen, are present in the gaseous source, said other gases may also be absorbed chemically and/or physically. This will be the case if alkanolamine is used as the absorbing agent.
It is well-known from the prior art that when O2 is present in the carbon dioxide-containing gaseous source, said O2 will be transferred into the alkanolamine-containing absorbing agent during the absorption procedure. As a consequence an unwanted degradation of alkanolamine as well as corrosion problems will occur due to the presence of O2. Therefore, removal of O2 from the absorbing agent will improve the efficiency of the absorption procedure.
Many prior art documents relate to this problem. EP 1 059 110 discloses a system for recovering absorbate such as carbon dioxide using an alkanolamine absorbent fluid, wherein the loaded absorbent is heated in a two step heating procedure prior to the separation of the absorbate from the absorbent, and wherein the loaded absorbent is deoxygenated after the first heating step and prior to the second heating step. The deoxygenation takes place by means of depressurisation.
In EP 1 061 045 a system for recovering absorbate such as carbon dioxide from an oxygen-containing mixture is described, wherein carbon dioxide is concentrated in an alkanolamine-containing absorption fluid, oxygen is separated from the absorption fluid, and carbon dioxide is steam stripped from the absorption fluid and recovered. In this system, the oxygen is separated from the absorption fluid by passing the carbon dioxide loaded absorbent comprising dissolved oxygen in countercurrent mass transfer contact with oxygen scavenging gas.
In other cases nitrogen oxides (also named NOx) may be present in addition to O2 in the gaseous source. These NOx gases will also be absorbed chemically and physically in the absorbing agent, when alkanolamine is used as the absorbing agent. When separating the carbon dioxide from the absorbing agent in a subsequent stripper process, part of the absorbed NOx will be released in the stripper off gas together with degradation products, especially acetaldehyde. The stripper off gas will further contain N2 and O2 in some amounts.
When producing food grade carbon dioxide or other carbon dioxide applications, where a high purity is required, these components must be removed from the stripper off gas in down stream equipment in order to obtain the required purity. Conventional technology available for removing NOx involves scrubbing, oxidation, adsorption and distillation.
Due to the chemical equilibrium: NO+½O2<−>NO2, the NOx composition (NO, NO2) will change during the purification procedure whenever changes in temperature, pressure and/or concentrations occur, and this makes it difficult to reduce the NOx content in the end product.
Hence, an object of the present invention is to provide a method for the recovery of high purity carbon dioxide, which is substantially free of nitrogen oxides.
The present inventor has surprisingly found that by introducing a flash column between the absorption column and the stripper the content of NOx in the stripper off gas can be markedly reduced.
This is due to the fact that when the equilibrium condition of the liquid leaving the absorption column is carefully changed just before feeding said liquid into the flash column a condition where said liquid is unsaturated in respect of O2 and NOx will occur, and consequently said gases will be transferred from the liquid phase into the gas phase during the flashing procedure. In this way substantially all O2 and the main part of NOx are removed from the liquid phase in the flash column and will therefore never reach the stripper.
In a next step, the liquid leaving the flash column is fed into the stripper column, in which the gases are separated from the absorbing agent. As a consequence of the very low amount of O2 reaching the stripper column, the concentration of O2 in the stripper off gas will be very low. Hence, in the stripper off gas the chemical equilibrium: NO+½O2<−>NO2, is shifted far to the left, and the traces of NOx present will mainly be in the form of NO. Therefore, the further purification procedure, which is required in order to remove said traces of NOx whenever high purity carbon dioxide is produced, is much easier and cost effective, because of the control of the above-mentioned chemical equilibrium.